Thermotropic liquid crystalline polymers (LCP's) are a well known class of polymeric compounds. Upon heating to a sufficiently high temperature, LCP's melt to form a liquid crystalline melt phase rather than an isotropic melt. Generally, LCP's consist of linear ("rigid rod") molecules that can line up to yield the desired liquid crystalline order. Thennotropic polymers also contain additional molecular units that break up the order enough to lower the melting point without destroying the liquid crystallinity. Examples of molecular units that lower the melting points of rigid rod polymers include isophthalate and 6-oxy-2-naphthoate. An alternate way to lower the melting temperature of a rigid rod polymer is to place a substituent on one or more of the rigid rod molecular units in the polymer, as for example using 2-phenyl-1,4-hydroquinone rather than unsubstituted 1,4-hydroquinone as a monomer.
2,5-Thiophenedicarboxylic acid (TDA) has a geometry that suggests that it may be useful for breaking up the molecular order in a rigid rod polymer. The carboxyl groups are at an angle of about 148.degree., intermediate between the linear geometry of the carboxyl groups of terephthalic acid (180.degree.) and the bent geometry of isophthalic acid (120.degree.). Only a single example of thermotropic liquid crystalline polymers that contain 2,5-dicarboxythiophene units has been reported (R. Cai, et al., Macromolecules (1992), 25, pp 563-568). The LCP's in that publication were derived from TDA and substituted 1,4-hydroquinone. Polymers of TDA and unsubstituted 1,4-hydroquinone were intractable, suggesting that the substituent on 1,4-ohydroquinone was necessary for liquid crystallinity to be achieved. It has also been reported that aromatic polyamides based partly on TDA form lyotropic LCP's (U.S. Pat. No. 5,266,677). Up to this time, it does not appear that TDA has been polymerized with aromatic diols and aromatic hydroxy acids to form liquid crystalline polymers.